In public service radio, it is common to superimpose control tones on audio signals. These control tones generally have sub-audible frequencies below 300 Hz, but may also be in the audible frequency range. These tones convey control commands and other information signals. The tones may be, for example, sent from one radio to another over an RF link or from a dispatcher console to a remote base station over land lines.
Tones are no longer used merely to remotely turn transmitters on and off and other basic control functions. Tones are performing an ever increasing variety of functions. Tones can be used to remotely control a base station or mobile transceiver receiver, monitor the operation and/or internal settings of a receiver, and send voting control information to central site controllers. It is impossible to predict how tones will be used in upcoming years, other than to say that tones will be used even more extensively than they are now used. For example, tones may be used to allow a mobile radio to link with a telephone connection at the base station and for many other radio operations.
Base stations, in particular, need to be able to encode and decode tones. It is often preferable to locate a base station near the antennas located on top of high buildings or on top of mountains. These inaccessible base stations are remotely controlled over land lines by one or more dispatcher consoles.
Tones are used to remotely control the base station. Furthermore, much of public service radio communications is between repeating base stations and remote mobile units and, thus, the base stations handle a large volume of the tone traffic in the system. Tone remote detection is typically used to remotely control base station systems. This coding allows a wide range of control functions to be implemented by remote tones. Tone remote detection is governed by industry standards that establish the format of tones and a library of their functions. The format for tone remote detection is a high volume secur-it tone (100 ms), a function tone (40 ms) and a hold tone if the function tone is a transmit function. The hold tone continues as long as a voice audio signal remains.
There are other kinds of tone codes used in public service radio. A common code is dual-tone multi-frequency (DTMF) coding for sending alphanumeric signals entered by a 12 or 16 pad keyboard. DTMF is generally referred to as "Touch Tone." DTMF coding is generally used to remotely turn on and off repeaters, especially the transmitter of the repeater. Type 90 tone is an older but still used tone code principally used to turn remote repeaters on and off. Type 90 coding uses single signals instead of the dual-frequency tones of DTMF.
Previously, base stations could not be easily adapted to accept new tones or handle an increasing volume of tone traffic. Prior base stations had separate hardware modules for each of the specific functions that were tone controlled. For example, a DTMF decoder module decoded the dual frequency alphanumeric tones of DTMF. Another module encoded DTMF signals. Similarly, tone remote control, voting tone generation, hold tone notching and Type 90 tone detection were each generated in separate modules. For example, the Ericsson-General Electric MASTR II Base Station Controller had separate plug in modules for tone remote control, tone generation and DTMF decoding.
Existing hardwired modules could not accept new tones without modification. Generally, another module was added to accommodate new tone types, assuming that the base station could be modified to accept new modules. Similarly, each module could only process a certain maximum amount of tone traffic. When the traffic exceeded the capacity of any one module, the base station had no ability to shift unused capacity in another module to the overloaded module. Accordingly, existing base stations do not have the capacity to handle new tones or large increases in the volume of any particular type of tone. Given the increasing use of tones, this inflexibility severely hampers the usefulness of existing base stations and the ease with which new tones can be added to the radio system.
The compartmentalized modules in prior base stations are bulky and complex. The numerous hardware modules increases the size and cost of the radio. Since each module has its own circuitry and logic units, and the modules interface with one another, the modules are complex and may be prone to failure.
Maintenance of modularized base station radios is time consuming and expensive. Modules are generally replaced when broken. The modules are not interchangeable with other modules in the base stations. Accordingly, a repair technician, who cannot generally predict which module is to be replaced before inspecting the radio, is required to carry a spare of every module in the radio or order a replacement for the broken module from the radio manufacture. It is impractical and prohibitively expensive to stock spare radio modules in every van of each repair technician. Thus, replacement modules are ordered from the manufacturer and the radio remains down (unusable) until the replacement module is received. Long down times for radios are abhorred, especially in public service radio systems which service police, fire squads, paramedics and other emergency units.
Maintaining and upgrading the performance of existing base stations generally requires the repair technician to visit the base station to make adjustments or add new components. Most prior base stations have adjustable potentiometers that are manually adjusted for each radio customer. Heat and other environmental factors affect the operation of these potentiometers. Accordingly, these potentiometers may need to be reset from time to time. The need for manual adjustments increases the need for and expense of maintenance of base station radios. Similarly, to upgrade a base station radio, such as to handle new tones, requires that a technician physically install new components into the radio and, possibly, manually adjust these new components. In short, prior base station radios lack flexibility and are expensive to maintain.
It has recently become known to provide remote mobile radio units with a digital signal processing board that plugs into the central controller board of the radio. U.S. Pat. No. 4,887,311, entitled "Radio With Options Board" issued on Dec. 12, 1989, to Terry Garner and Robert Dixon (the Garner Patent). The Garner Patent discloses an optional digital processor that interfaces with the control microprocessor for the radio. This digital processor allows the radio to handle several additional tone controlled functions. The digital processor receives processed RF audio from the radio's audio processor. The digital processor issues commands to the control microprocessor to implement these new tone controlled functions.
The digital processor disclosed in the Garner Patent works well with mobile radio units that communicate solely over an RF link but not over land or telephone lines. Moreover, the relatively low volume of traffic flowing through a mobile radio unit allows the existing audio processor to handle an add on digital signal processor. Finally, the radio control architecture disclosed in the Garner Patent is well suited to a mobile radio unit, but not to a base station. While the Garner Patent discloses a digital processor for mobile units, there remained a need for a flexible tone detection, filtering and encoding device for base stations having multiple audio inputs and handling a high volume of tone traffic.
This need for digital processing flexibility in a base station is satisfied by the present invention. We have developed a base station having a universal digital signal processing (DSP) module that is fully programmable. In the preferred embodiment, the DSP module is a piggy-back board that attaches to the central controller board (control shelf) of the base station radio. The DSP module preforms the tone decoding, filtering and generation previously performed by separate hardware modules. The DSP module has digital potentiometers that can be remotely programmed. All of the operating parameters of the DSP board can be programmed. Thus, the operating personality (e.g. operating parameters unique to the customer), ability to handle tones and maintenance adjustments can be programmed or reprogrammed into the DSP module without altering the hardware of the base station.
Moreover, the DSP board can be remotely monitored and programmed. For example, a technician can communicate directly with the radio board via a telephone interface, receive information regarding current settings and operation of the DSP board, and reset the DSP board including its potentiometers over the telephone. Accordingly, the technician can remotely diagnose and often remotely repair the base station. If the technician is required to physically visit the radio, he need only bring a single substitute DSP board, instead of the many hardware modules previously required.
A universal DSP board and digital signal processor replaces the various hardware modules previously required for signal processing. The DSP board is compact, inexpensive and flexible compared to the numerous hardwired modules used for tone control in prior base stations.
In the preferred embodiment, the DSP board includes two coder-decoder (codec) processors to handle the conversion of audio between analog and digital signals. Audio signals are received and transmitted over RF links, and over land lines and telephone lines. The DSP processor operates on the digitized audio signals through serial interfaces between each of the codecs.
The DSP board module interfaces with the controller board of the base station. A parallel communication interface including a dual port random access memory (RAM) device allows the controller board to communicate with the DSP microprocessor. The central controller can set the operating parameters of the DSP board, issue commands to the DSP microprocessor such as tone generation, and receive information derived from the audio signals by the DSP microprocessor.
The DSP board fundamentally handles tone detection, tone filtering and tone generation. These functions may be performed simultaneously. Each function may be enabled or disabled. Additionally, functions can be programmed into the DSP processor. Similarly, the controller can alter operating parameters for the DSP board, such as the potentiometer settings and codec channel assignments.
The DSP board of the present invention handles detection and decoding for DTMF, Type 90 single tones, and tone remote. The detection schemes employed by the DSP microprocessor all employ the Goertzel Algorithm to extract tone frequency information from the digitized audio signal. In addition, the DTMF decoding scheme includes several post-tests to prevent false detection of tones. Similarly, the tone remote decoding scheme has an energy-based algorithm that discriminates between tones having the same frequency such as secur-it, function and hold tones. When it recognizes a new valid tone, the DSP microprocessor passes the tone information it decodes to the controller microprocessor.
In the present invention, the DSP board provides tone filtering. It is known to use notch filters to remove a control tone superimposed on a audio signal. This filtering technique is known as "hold-tone filtering." Notch filters have previously been separate hardware modules in the radio. Incorporating the notch filter as a programmed function of the DSP microprocessor reduces the number of modules in the radio controller and allows for remote digital reconfiguration of the filter by reprogramming the microprocessor.
Similarly, in some applications, certain audio bands are notched out of the signal to prevent corruption of the control tone that is to be superimposed on the audio signal. This notching is "anti-false filtering" and is predominately used in conjunction with the 2-wire remote line interface of the base station.
Tone generation enables a base station to send control signals to external devices. The DSP board, in the preferred embodiment, can generate any tone having a frequency up to 3.2 KHz. The DSP processor is programmed with an algorithm used to generate tones. This algorithm includes sinusoidal look-up techniques to enhance the precision and efficiency of the tone calculations. By using this powerful and flexible tone generation algorithm, a single DSP processor has been substituted for several prior tone generation hardware modules.
The DSP board can function as an all digital, programmable compressor. Line audio inputs from several sources are filtered, converted to digital, and compressed by the DSP processor. The processor executes an audio compression program that selects and applies the appropriate gain to the digital signal. The compressed signal leaves the processor, is converted to analog form, filtered and routed to a variety of outlets such as an RF power transmitter or line links.
As a digital programable compressor, the DSP board eliminates hum and noise on the line input. In the preferred embodiment, the compressor outputs zero (0) voltage if the line input voltage is below a threshold value. Thus, the usual noise and hum from audio lines are eliminated.
The DSP compressor acts as a hard-limiter to prevent the compressor output signal amplitude from exceeding selected values. Moreover, the DSP processor provides digital control of the various threshold levels applied by the compressor. The DSP processor can be used to implement a software potentiometer. Accordingly, the compressor thresholds can be modified by resetting these software potentiometers in the DSP processor.
It is an objective of the present invention to improve the performance and flexibility of base stations by centralizing most of the tone decoding, filtering, compression and tone generation functions of a radio base station in a single DSP board. In addition, it is an object to reduce the size of base station radios, lower their cost, and simplify their maintenance and repair.
Furthermore, it is an object of the present invention to provide a universal digital signal processing board that can be easily reconfigured to meet various base station requirements. This flexibility allows the base station to be (re)configured either on site or remotely to promptly satisfy customer requirements or to handle new applications of control tones.